As an image processing device, there has been one that generates output image data by performing data processing of image input data, depending on not only current image data input but also a state resulting from processing of past image data input. In the above-described image processing device, the data indicating the state resulting from the processing is generated as state image data to be temporarily stored in an image memory called a frame memory, and is used for data processing of a next frame (e.g., see Patent Document 1).
On the other hand, with recent rapid increase in definition and processing speed of image data, an image data amount to be processed by a device and a system has been explosively increasing. According to this, a required frame memory size and data transfer capability to the frame memory have been increasing as well, so that implement in a practical circuit has become difficult. Thus, reduction in an amount of data stored in the frame memory is very important.
As a method for reducing the amount of data, a technique of encoding and compressing the image data to be stored in the frame memory is generally employed. FIG. 5 is a block diagram showing a schematic configuration of a typical image processing device that generates state image data from input image data and compresses the state image data to store the same in the frame memory, and generates output image data, using the decompressed state image data in processing of a next frame.
Conventionally, for image compression to the frame memory, various methods have been proposed and disclosed as patents. Broadly speaking, as the image compression technique, there are a reversible compression technique and an irreversible compression technique. The former is also called lossless compression, and when data is compressed and decompressed, the original data can be restored without loss of an information amount. The latter is called lossy compression, and even when the data is compressed and decompressed, there is no assurance that the original data can be restored, and an error occurs between the original data and the data after compression/decompression. This is referred to as a compressibility error. Though the compressibility error is 0, generally in the lossless compression, the lossy compression is used when the frame memory amount is desired to be largely reduced since the lossy compression is high in compressibility. Patent Document 2 discloses, as one technique of the lossy compression, a device in which encoding means with smaller signal deterioration is selected from means of encoding image input data (PCM processing) and means of differentially encoding the image input data (DPCM processing) to reduce and eliminate image quality deterioration. An image processing device, which the present invention targets, can be configured as shown in FIG. 5, for example, using the device disclosed in Patent Document 2 as image compression means and image decompression means.
In the image processing device shown in FIG. 5, the compressibility error occurs when the state image data is compressed and stored in the frame memory, and the state image data stored in the frame memory is decompressed at the time of data processing of the next frame. The compressibility error is decided in accordance with a processing method of the image compression means and the image decompression means, and is preferably smaller, because precise processing at a subsequent stage can be expected.